1. Field of the Invention
The present invention relates to a resonant converter that operates in a burst mode according to a load. When a load connected to an output terminal of the converter is low, the converter stops performing a switching operation for a time and restarts the operation after a delay. The above operations can be performed repeatedly.
2. Description of the Related Art
When a switching frequency of a converter exceeds a predetermined threshold value, the converter may switch to a burst mode operation. As a load connected to the converter decreases, the switching frequency may increase to maintain an output voltage. When the switching frequency increases, a switching loss may increase. The converter may switch to the burst mode operation to reduce the switching losses. The threshold value of the switching frequency can be varied according to an input voltage. However, additional circuit elements are required to vary the threshold value according to the input voltage.
FIG. 1 is a diagram representing a relationship between a gain and a switching frequency according to a load. The gain is a ratio of an input voltage and an output voltage of a converter.
When the load decreases, the switching frequency can increase to maintain the gain level. In addition, when an input voltage varies, the gain and the switching frequency can also vary. In detail, the converter can be driven to maintain the output voltage at a predetermined level regardless of the input voltage. This can be achieved by reducing the gain when the input voltage increases. In other cases the switching frequency can be increased when the gain decreases under the same load condition. In some cases, since the switching frequency varies according to a variation of the input voltage, the threshold switching frequency may need to be varied to control the onset of the burst mode.
Further, additional circuitry may be necessary to determine the switching frequency and to compare the switching frequency and the threshold value switching frequency while the converter is driven.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form prior art that is already known in this country to a person of ordinary skill in the art.